Densitometers are well known devices which scan a sample and provide an output signal or graphical display indicative of the optical density, transmittancy, absorption or the like of the scanned sample. One well known use of the densitometer involves scanning a sample of blood which has been prepared by the electrophoresis process. Electrophoresis of blood samples isolates the various proteins in the blood, known as albumim, alpha-one globulin, alpha-two globulin, beta-globulin and gamma-globulin. The electrophoresis technique separates these proteins from each other, following which the sample is scanned by an optical densitometer pick-up. Each of the proteins exhibits a different light absorption characteristic or pattern and the light absorption patterns are graphically displayed by the densitometer to indicate the presence and quantity of each of these proteins.
In optical density analysis, the amount of light passing through the sample is an inverse logarithmic function of the optical density of the sample. Thus, if the optical density of the sample is increased from 1 OD (optical density) to 2 OD, the transmitted light is reduced by a factor of 10. The light transmitted through a sample falls on a photo-responsive element of the pick-up head which generates electrical signals having a current proportional to the amount of transmitted light. The current output of the photo-responsive element is, therefore also a logarithmic function of the optical density which is then converted into analog or time-varying signals directly proportional to the optical density patterns of the scanned sample. The analog signals are employed to drive a graphic display unit to provide a permanent curve or record of the optical density pattern.
In addition to scanning densitometry which measures the emergent radiation passing through a sample as a measure of the sample's density either by transmittance or absorbance measurements, fluorescent densitometry has gained wide acceptance in clinical laboratories. Some materials, when excited by energy of a short wavelength, re-emit light of a longer wavelength. The ultraviolet energy is used only to excite the fluorescent material and, unlike transmission densitometry, is not the light used for quantitation. The only light detected and measured in fluorescent densitometry is the light emitted by the sample and the relationship between the emitted light of the sample and its concentration is linear rather than logarithmic as it is with transmission densitometry. Hence, with fluorescent techniques, a linear rather than a logarithmic amplifier must be used for measurement purposes.
Finally, it is possible to analyze the optical density patterns of the sample by measuring the amount of light which is reflected from the sample, and in this case a linear amplifier is also employed for measurement purposes.
In any event, the electrical analog signals generated by the photo-responsive elements, when graphically displayed, exhibit a series of peaks and valleys. In the analysis of blood, the area under the optical density curve and bounded by two adjacent valleys separated by one peak, is representative of the quantity of each protein in the sample and is referred to as the sample fraction. Of primary importance is the relative percentage of each protein and the selection of these fraction boundaries i.e., the precise location of these valleys is somewhat arbitrary and results in inaccurate analysis of the blood sample. This problem is not unique to evaluation of blood samples but is common to optical and magnetic density evaluations, and, in fact, to all evaluations of analog data.
The incorporation of microprocessors in densitometer instruments has reduced operator involvement by automating instrument control, signal digitizing, sample storage, pattern interpretation, sample recall and reconstruction of the analog signals for display on a CRT and/or printer. U.S. Pat. No. 4,242,730 issued Dec. 30, 1980 to Golias, et al is typical of recent prior art, microprocessor controlled densitometers which are highly automated. The system shown in this prior patent employs a computer controlled carriage for moving the sample relative to an optical detector in order to scan the sample. Scanning of the sample results in the generation of an electrical analog signal which is function of the optical density of scanned sample. The electrical analog signal is processed and converted into digital sample data for storage in a memory. Under microprocessor control, a CRT displays the analog waveform pattern which is representative of the optical density patterns and which has been reconstructed from the stored digital sample data. While the waveform pattern is displayed on the CRT, the operator may visually inspect and edit the waveform pattern. This prior system also provided for normalizing the electrical analog signal in order to produce full scale readings. This normalization was performed by digital calculations on the digitized sample data stored in memory. In order to achieve normalization for the relatively wide range of electrical analog signals typically experienced in a clinical laboratory setting, ranging from a relatively low level fluorescent pattern, to a relatively high level serum protein pattern, this prior system included a set of manual controls for adjusting the analog gain depending upon the type of pattern to be scanned, prior to scanning the sample. However, since the amplitude of the analog signal being sampled was not displayed for the operator, the operator often neglected to use the manual gain controls. As a result, the operator would not be aware of, for example, a low gain setting that may have been left from a previous scan that required the gain to be set low because of a high level pattern previously scanned. The prior system employed a twelve bit analog to digital (A/D) converter to convert the input analog signal to digital format for storage. This arrangement provided sufficient accuracy and resolution for all patterns provided that the analog gain was properly set. However, if the gain was set for a high level pattern, some of the low level patterns produced a signal of less than 1/16th of full scale in the event that the gain control had not been reset for the lower level signal. For example, a Serum Protein type pattern has a normal intensity of up to 2.5 optical density (OD), while the Cholesterol type of pattern has a normal value of less than 0.3 OD. If the last pattern scanned was serum protein and the analog gain control had not been reset and the operator then scanned a Cholesterol pattern, a relatively low strength analog signal is digitized and normalized. Since there are normally always variations from scan to scan resulting from system variables such as pattern skew, carriage speed variations, temperature changes, auto zero changes, their effects can cause variations outside the acceptable range of limits when the gain is not readjusted.